930 REV. G. HENSLOW ON THE ORIGIN OF PLANT-STRUCTURES 
aphyllous species of Veronica live at an elevation of 7000 feet, 
V. thujioides is found at a lower but still high elevation. 
Similarly Thujas (Biotas) are trees and shrubs of considerable 
elevation. So many coincidences here offer the same grounds 
in support of the contention that similar causes have brought 
about similar results and produced these mimetic forms in 
genera of widely different orders. Thus Zamarix in Africa may 
be said to represent Halovylon Ammodendron of Beluchistan and 
the Oriental steppes (Aitchison and Grisebach), Casuarina of 
Australia, the Thujas of Japan and California, Veronica thu- 
jiotdes of New Zealand, &ce. 
V. Succulent Plants. 
Although spinescence and hairiness are the prevailing features 
in the desert plants near Cairo, some few are decidedly succulent, 
as the species of Zygophyllum. That this feature is one of the 
direct results of the intense heat (probably influenced by the 
presence of salts in the soil), inducing the formation of a thick 
cuticle, which, in turn, involves the retention of water and the 
development of succulent aquiferous tissues, I think cannot be 
doubted. The presence of salts has been proved by M. Lesage 
to be the immediate cause of succulency in maritime plants 
of temperate climates*; and he succeeded in making plants 
succulent which are not so ordinarily. On the other hand, the 
structure of “ rock plants,” such as Sedums, Haworthias, &e., is 
correlated with their arid and stony surroundings (probably 
without the aid of salts), and is obviously one of the many 
adaptations for the storage of water. 
That the succulency is due to the direct action of the environ- 
ment, is shown by the results of experiments in which the 
normal succulency is made to disappear when a new combination 
of surrounding conditions is supplied to the plant. Thus 
M. Battandier T cultivated Sedum Clusianum, and the leaves 
at once began to assume a flatter character, and he remarks as 
a coincidence that the two species S. stellatum and S. tuberosum, 
which are not rupicolous in France, but inhabit wet places, 
have flat leaves. On the other hand, I have found the leaves of 
S. stellatum growing in cracks between flat slabs of rock in Malta, 
* The succulency of several members of the Chenopodiacee which frequent 
saline marshes and deserts may be now attributed to the same cause. 
t Bull. Soc, Bot. de Fr. 1887, p. 191. 
